You can use Magic Jellybean or Belarc Advisor to extract the key from your current installation.
Your computer may come with recovery software. You can use that to re-install Windows on the SSD.
You can also download a legitimate copy of Windows 8 from here (http://windows.microsoft.com/en-CA/windows-8/upgrade-product-key-only) if you have the product key.
If that link doesn't work, you can use this article to get to the legal download site: http://www.howtogeek.com/186775/how-to-download-windows-7-8-and-8.1-installation-media-legally/